The present invention relates to a magneto-optical recording and reproducing method and a recording and reproducing apparatus therefor, to be carried out by using a magneto-optical recording medium. In particular, the present invention relates to a magneto-optical recording and reproducing method and a recording and reproducing apparatus therefor, based on the use of a novel reproducing technique which has not been used in any conventional magneto-optical recording and reproduction process.
In order to realize a more advanced high density process carried out on a magneto-optical recording medium, a technique attracts attention, in which the magneto-optical recording medium is irradiated with a reproducing laser beam while applying an external magnetic field during reproduction to obtain a reproduced signal.
The magnetically induced super resolution technique has been suggested, as described, for example, in Journal of Magnetic Society of Japan, Vol. 17, Supplement No. S1, p. 201 (1993). This technique improves the magnetic film of the magneto-optical recording medium, and it utilizes the occurrence of the temperature distribution in a light spot upon irradiation with a reproducing light beam. Thus, a magnetic mask is generated in the spot so that the effective spot diameter, which contributes to signal reproduction, is reduced. The use of this technique makes it possible to improve the reproducing resolving power without reducing the optical spot diameter.
Another technique for amplifying a minute reproduction signal has been also suggested, as disclosed, for example, in Japanese Patent Application Laid-Open No. 1-143041, in which the magnetic domain is magnified by applying an external magnetic field during reproduction. Japanese Patent Application Laid-Open No. 6-259823 also discloses a technique for amplifying the magnetic domain.
The use of the magnetically induced super resolution technique and the magnetic domain-magnifying reproduction technique makes it possible to mutually distinguish and reproduce a plurality of minute magnetic domains existing in the reproducing light spot. However, since such magnetic domains are subjected to high density recording, the recording clock cycle is short. When such signals are reproduced, it is feared that C/N is deteriorated due to occurrence of interference between waveforms. For this reason, a technique is further required to improve C/N when the magnetic domains, which are subjected to high density recording, are reproduced by using the techniques as described above.
In the case of the conventional techniques described above, the reproduced signal, which is obtained upon reproduction, corresponds to information recorded on the magneto-optical recording medium, either when the magnetically induced super resolution technique is used, or when the minute reproduced signal is amplified by applying the external magnetic field during reproduction to magnify the magnetic domain, because of the following reason. That is, it has been premised that the recorded information should be faithfully reproduced.
However, it is not necessarily demanded that all of recorded information is faithfully reproduced, depending on the use or application of an application system to be constructed. In some cases, it is intended to obtain different pieces of reproduced information depending on the use and the purpose. For example, in a certain application system in relation to any security use, there may be a case in which only information, that is recorded on a specified portion of a magneto-optical recording medium, is intended to be reproduced when information recorded on the magneto-optical recording medium is reproduced. Similarly, in an application system in relation to any security use or any cipher description use, there may be a case in which it is intended to directly obtain a reproduced signal by converting information with a specified function, when information recorded on the magneto-optical recording medium is reproduced. Further, depending on the use of the application system, it is desired to perform direct reproduction while thinning out information at specified intervals, when information recorded on the magneto-optical recording medium is reproduced.
An object of the present invention is to provide a magneto-optical reproducing method and a reproducing apparatus therefor, for obtaining different pieces of reproduced information depending on the use and the purpose, without faithfully reproducing all of recorded information, when information recorded on a magneto-optical recording medium is reproduced. Another object of the present invention is to provide a magneto-optical reproducing method and a reproducing apparatus therefor, which make it possible to reproduce, with high C/N, information subjected to high density recording on a magneto-optical recording medium.
According to a first aspect of the present invention, there is provided a method for reproducing information recorded on a magneto-optical recording medium while applying an external magnetic field to the magneto-optical recording medium, the method comprising the step of:
applying the external magnetic field including different application patterns to an identical recording area on the magneto-optical recording medium so that different pieces of information corresponding to the application patterns are reproduced from the identical recording area. The application pattern is represented as a function H(x) of the magnetic field intensity with respect to the position x on the magneto-optical recording medium. It is possible to construct a system in which the function H(x) is used as a password to obtain information, and only a person, who knows the function H(x), is able to access specified information capable of being reproduced with the function H(x).
According to a second aspect of the present invention, there is provided a method for reproducing information recorded on a magneto-optical recording medium while radiating a reproducing light beam onto the magneto-optical recording medium, the method comprising the step of:
radiating the reproducing light beam including different radiation patterns onto an identical recording position on the magneto-optical recording medium so that different pieces of information corresponding to the radiation patterns are reproduced from the identical recording position.
According to a third aspect of the present invention, there is provided a magneto-optical reproducing apparatus for reproducing recorded information by radiating a reproducing light beam onto a magneto-optical recording medium, the apparatus comprising:
a magnetic field unit for applying an external magnetic field to the magneto-optical recording medium during reproduction;
a means for selecting a specified external magnetic field application pattern from a plurality of external magnetic field application patterns; and
a driving unit for driving the magnetic field unit in accordance with the selected external magnetic field application pattern, wherein the information recorded on the magneto-optical recording medium is reproduced while applying the magnetic field to the magneto-optical recording medium in accordance with the specified external magnetic field application pattern to make it possible to obtain specified information based on the pattern.
According to a fourth aspect of the present invention, there is provided a magneto-optical reproducing apparatus for reproducing recorded information by radiating a reproducing light beam onto a magneto-optical recording medium, the apparatus comprising:
a light source for radiating the reproducing light beam onto the magneto-optical recording medium during reproduction;
a means for selecting a specified radiation pattern from a plurality of radiation patterns; and
a driving unit for driving the light source in accordance with the selected specified pattern, wherein:
the information recorded on the magneto-optical recording medium is reproduced while radiating the reproducing light beam onto the magneto-optical recording medium in accordance with the specified radiation pattern to make it possible to obtain specified information based on the pattern.
The principle of operation of the magneto-optical recording and reproducing method according to the present invention will be explained below. Recording is performed on the magneto-optical recording medium by magnetizing the recording layer in an area intended to perform recording, in a desired direction. Reproduction is performed by detecting the magneto-optical effect such as the Kerr rotation angle and the Faraday rotation angle. In an ordinary reproduction process, it is desired to stably obtain a reproduced signal faithfully corresponding to the recorded magnetization state. In order to stably obtain the reproduced signal faithfully corresponding to the recorded magnetization state, it is desired to obtain a reproduced signal having a stable shape, regardless of slight fluctuation of the radiation state of the laser beam to be radiated during the reproduction. When the external magnetic field is applied during the reproduction in accordance with, for example, the magnetically induced super resolution, it is desired to obtain a reproduced signal having a stable shape, regardless of slight fluctuation of the application state of the external magnetic field.
On the contrary, the present invention actively utilizes the change of the shape of the reproduced signal by changing the application pattern of the external magnetic field applied during the reproduction, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof. Even when an identical magnetization state is recorded on the recording layer of the magneto-optical recording medium, it is possible to obtain a plurality of reproduced signals having different shapes by changing the application pattern of the external magnetic field, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof, because of the following reason. That is, the magnetization state of the reading layer, which directly contributes to the reproduced signal, can be changed by changing the application pattern of the external magnetic field, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof. It is especially preferable to use a magneto-optical recording medium comprising a plurality of stacked layers including two layers of a recording layer and a reading layer, the magneto-optical recording medium being constructed such that the magnetization state of the reading layer depends on the magnetization state of the recording layer and the temperature distribution caused by irradiation with the reproducing laser beam. The use of such a magneto-optical recording medium makes it possible to obtain a plurality of reproduced signals having different shapes by changing the application pattern of the external magnetic field, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof. In other words, the present invention is preferably applied to perform the recording and the reproduction on the magneto-optical recording medium comprising a plurality of stacked layers including at least the recording layer and the reading layer, and the present invention is especially preferably applied to perform the recording and the reproduction on the magneto-optical recording medium capable of performing the magnetically induced super resolution or the magnification of the magnetic domain.